Can someone share a link on "how to map just 1 side of a box" and not UV map the entire box?
UV map will contains all the faces anyway (this is structurally how the data is). You can select one face and unwrap it. It will take all the surface normally. But the other faces will exist and all be at 0, 0.

As shown here:



Note: 

If you've a previously created UV map, when creating a new one selecting only one face, the other faces will keep the UV of the previously created UV map.